Nucleic acid sequencing-by-synthesis has the potential to revolutionize the understanding of biological structure and function. Traditional sequencing technologies rely on amplification of sample-based nucleic acids and/or the use of electrophoretic gels in order to obtain sequence information. More recently, single molecule sequencing has been proposed as a way to obtain high-throughput sequence information that is not subject to amplification bias. See, Braslavsky, Proc. Natl. Acad. Sci. USA 100: 3960-64 (2003).
Sequencing-by-synthesis involves the template-dependent addition of nucleotides to a support-bound template/primer duplex. The added nucleotides are labeled in a manner such that their incorporation into the primer can be detected. A challenge that has arisen in single molecule sequencing involves the ability to sequence through homopolymer regions (i.e., portions of the template that contain consecutive identical nucleotides). Often the number of bases present in a homopolymer region is important from the point of view of genetic function. As most polymerases used in sequencing-by-synthesis reactions are highly-processive, they tend to add bases continuously as the polymerase traverses a homopolymer region. Most detectable labels used in sequencing reactions do not discriminate between more than two consecutive incorporations. Thus, a homopolymer region will be reported as a single, or sometimes a double, incorporation without the resolution necessary to determine the exact number of bases present in the homopolymer.
One solution to the problem of determining the number of bases present in a homopolymer is proposed in co-owned U.S. Pat. No. 7,169,560. That method involves controlling the kinetics of the incorporation reaction such that, on average, only a predetermined number of bases are incorporated in any given reaction cycle. The present invention provides an alternative solution to this problem.